Tino wrote:18x9... thats way too thick
Are you serious? There's really not such a thing as long as they fit. I think Options Auto Salon and some Japanese wheel manufacturers RECOMMEND a 9" wheel in the back.
I've got 17" x 8" all around and I've got room to spare....running 245/40/17's and that's the narrowest I would ever go in the rear...looks too damn mean to ever go back...not to mention I would never get traction with anything less.
The brand of tire you run and the model will determine what width you should use...
http://www.tirerack.com has some good information on their tires...they tell what the recommended wheel width is for each tire and size and ALSO give you a safe range which is usually 1 to 2 inches. I believe it's the measuring wheel width that is the recommended width. My best guess is a 275, although a 255 would probably fit nicely as well.
As far as wheels go, it's all about offset. I've had two sets of 8" wheels on my car...one didn't even come close to clearing the Brembos and had to have huge spacers. This looked like s***, so I put a set of stock Cobra wheels with an almost perfect offset and got down to a 5mm spacer and they look great.